Conversion method

ABSTRACT

A process is described for the preparation of water-soluble cellulose hydrolysis product. The process comprise admixing cellulose with an ionic liquid capable of solvating or dissolving at least some of the cellulose, said ionic liquid being a compound comprises solely of cations and anions and which exists in a liquid state at a temperature at or below 150° C., and in which the anions are selected from halide and cyanate; and treating the resulting solvate or solution with an acid in the presence of water, said acid having a pKa in water of less than 2 at 25° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 12/303,358, which inturn is a national stage application of International Patent ApplicationNo. PCT/GB07/01859, filed May 18, 2007, which in turn claims priority toEuropean Patent Application No. 06252728.8, filed May 26, 2006.

The present invention relates to a method of hydrolysing cellulose togenerate water soluble monosaccharide, disaccharide and oligosaccharidederivatives thereof.

Cellulose is the most abundant biorenewable material on earth. Celluloseconsists of polydisperse linear polymeric chains formed by repeatedconnection of beta-D-glucose building blocks through a 1-4 glycosidiclinkage. These linear polymer chains form hydrogen-bonded supramolecularstructures that are insoluble in water and most common organic solvents.It is known that hydrolysis of cellulose generates monosaccharide,disaccharide and oligosaccharide products, with glucose usually beingthe main hydrolysis product. Such products are capable of beingfermented to generate alcohols for use as a fuel or a component of afuel.

Glucose in particular is an important intermediate for fermentation toethanol and other chemicals; therefore, saccharification of cellulose isof interest in the development of biofuels.

Chemical, enzymatic, microbiological and macrobiological catalysts canbe used to accelerate the hydrolysis of cellulose under conditionsselected to be thermodynamically favourable to product formation.Chemical and enzymatic hydrolysis of cellulose is discussed in “TheEncyclopaedia of Polymer Science and Technology”, 2nd Ed, J. I.Kroschwitz (Ed in Chief), Wiley (New York), 1985. Thus, cellulose may behydrolysed using cellulolytic enzymes (cellulase) or harvestedfilamentous fungi such as Trichoderma sp. However, hydrolysing celluloseby chemical methods presents many problems. In general, such methodshave involved one of two approaches: dilute acid treatment at hightemperatures and pressures (>100° C.) and/or concentrated acidpre-treatment, as described in “Cellulose to Ethanol”: A GeneralReview”, P. C. Badger, in “Trends in New Crops and New Uses”, J. Janickand A. Whipkey (Eds), ASHS Press, Alexandria Va., 2002, 17-21. Diluteacid processes are conducted at high temperature under pressure (forexample, using 1% sulphuric acid at 237° C.). Concentrated acidprocessing typically starts with an initial acid concentration of 10%which is raised to 70% through dewatering at 100° C. and ambientpressure.

Because of the low yields and/or extreme conditions associated withthese known processes, there remains the need for an improved method ofhydrolysing cellulose by chemical means. Specifically, there is a needfor a relatively rapid reaction which may be carried out underrelatively mild conditions to give an adequately high conversion tosugars.

It is known that cellulose can be dissolved in certain ionic liquids.For example, U.S. Pat. No. 6,824,599 discloses that cellulose can bedissolved in a hydrophilic ionic liquid in the substantial absence ofwater or a nitrogen-containing base to form an admixture, which is thenagitated until dissolution is complete, while WO 2005/017001 disclosesthat wood, straw and other natural lignocellulosic materials can bedissolved in certain ionic liquids under microwave irradiation and/orunder pressure. The present inventors have now found that certain ionicliquids containing a certain specific anion can be used in a process forthe hydrolysis of cellulose.

Accordingly, the present invention provides a process for thepreparation of water-soluble cellulose hydrolysis products, whichcomprises admixing cellulose with an ionic liquid capable of solvatingor dissolving at least some of the cellulose, said ionic liquid being acompound comprised solely of cations and anions and which exists in aliquid state at a temperature at or below 150° C., and in which theanions are selected from halide and cyanate; and treating the resultingsolvate or solution with an acid in the presence of water, said acidhaving a pKa in water of less than 2 at 25° C.

A preferred embodiment of the process of the invention provides aprocess for the preparation of water-soluble cellulose hydrolysisproducts, which comprises admixing cellulose with an ionic liquid inwhich the cellulose has at least some solubility, said ionic liquidbeing a compound comprised solely of cations and anions and which existsin a liquid state at a temperature at or below 150° C., and in which theanions are selected from halide and cyanate; and treating the resultingsolution with an acid in the presence of water, said acid having a pKain water of less than 2 at 25° C.

Throughout this specification and claims, except where the contextrequires otherwise, the term “cellulose” should be understood to includeboth cellulose itself and cellulose-containing material, either in rawor purified form. The cellulose that is to be hydrolysed may be eithercellulose which has been refined to any desired degree, or it may be rawor partially-treated cellulosic material, such as cellulosic biomass ormunicipal waste. It may be used in any form that is amenable to beingwetted by a liquid. For example, the cellulose may be present in, orderived from, wood (particularly, wood chips and wood pulp), cotton,rayon, cellulose acetate, paper, linters, grasses such as corn stover orswitch grass, or bagasse (sugar cane residue).

The acid used in the process of the invention is a strong acid, having apKa in water of less than 2, preferably less than 1, preferably 0 orless, at 25° C. An acid with a pKa of 0 is fully dissociated in water,and such acids are preferred for use in the present invention. The acidsused in the invention are of the Brönsted (or protonic) type. Suitableacids include for example hydrogen halides, sulfuric acid, nitric acid,halosulfonic acids, tetrafluoroboric acid, heteropolyacids, aryl- andalkyl-sulfonic acids, and halogenated alkyl- and arylsulfonic acids.Examples of suitable acids include, for example, p-toluenesulfonic acid,trifluoromethanesulfonic acid (triflic acid), trichloromethanesulfonicacid, hydrochloric acid, hydrobromic acid, hydriodic acid,tetrafluoroboric acid, and sulfuric acid. Preferred acids are sulfuricacid and hydrochloric acid.

The acid may be added in aqueous form, for example dilute aqueous form,or if desired may be anhydrous. Some water is needed in order for thehydrolysis reaction to occur as explained below, and this may either bepresent in the reaction mixture and/or added along with the acid. Amixture of acids may be used provided that at least one acid has therequired acid strength, or the mixture has the required acid strength.In addition to the protonic acid, a Lewis acid may also be added to thereaction mixture if desired.

Suitable Lewis acids include metal salts of strong protic acids (pKaless than about 0), in which the metal is for example lithium,potassium, magnesium, zinc, copper, aluminum, tin, antimony, iron,nickel or lanthanum. Suitable examples of such salts include, forexample, metal halides, for example aluminum (III) chloride, gallium(III) chloride, indium (III) chloride and zinc (II) chloride; triflates,for example lithium triflate, sodium triflate, magnesium triflate, zinctriflate, aluminum triflate, tin (II) triflate, and copper (II)triflate; tetrafluoroborates, for example zinc (II) tetrafluoroborate,silver (II) tetrafluoroborate, iron (II) tetrafluoroborate, and nickel(II) tetrafluoroborate; and sulfonates, for example zincp-toluenesulfonate.

Preferably, a catalytic amount of the acid is used. For example, theconcentration of the acid in the reaction mixture may be from 0.1-10 wt%. If the reaction mixture before addition of the acid contains anybasic material, some of the acid initially added will be neutralised,and sufficient acid needs to be added taking this into account.

The process of the invention is suitably carried out until a desiredproportion of the cellulose is converted into water soluble derivatives.Suitably, the treatment with the acid proceeds for up to 96 hours,preferably less than 24 hours, more preferably less than 5 hours, andmost preferably less than 1 hour.

The process of the invention may be carried out at any suitabletemperature. Admixture of the cellulose with the ionic liquid must, ofcourse, be carried out at a temperature at which the ionic liquid is infact liquid. Subsequent reaction with the acid may if desired beaccelerated by heating; for example, the reaction may be carried out ata temperature in the range 50 to 200° C., preferably 70 to 150° C., forexample 90 to 95° C. Heating can be accomplished by any suitable method,for example using conventional thermal methods, microwave heating oremploying other sources such as ultrasound or infrared radiation.Preferably the reaction is carried out under atmospheric pressure.

The ionic liquid used in the process of the invention is a compound thatconsists of cations and anions and that is in a liquid state at atemperature at or below 150° C., preferably at or below 100° C., forexample in the range −100° C. to 150° C., preferably −10 to 100° C. Itis necessary that the ionic liquid should be capable of dissolving atleast some of the cellulose, or should be capable of solvating at leastsome of the cellulose. When the cellulose is used in the form ofbiomass, solvation generally leads to swelling of the biomass, and thismay be a preferred mode of operation when treating biomass.

Alternatively, an ionic liquid may be selected in which the cellulose isreadily soluble. On admixture of the cellulose with the ionic liquid,conditions may be chosen such that the cellulose becomes solvated by theionic liquid; substantially all of the cellulose dissolves to form ahomogeneous solution; or some cellulose dissolves while some remainsundissolved. Particularly in the latter case, residual solid materialmay if desired be removed from the solution of cellulose in the ionicliquid by any suitable method. Alternatively, the mixture may be usedwithout further treatment. Suitably, an ionic liquid is selected inwhich simple solvation or dissolution takes place—i.e. solvation ordissolution without cellulose derivatisation. Naturally, the ionicliquid should be adequately inert in the presence of the strong acidused in the process of the invention; ionic liquids containing basicgroups which would neutralise the acid are undesirable.

The anion of the ionic liquid must be halide (chloride, bromide, oriodide) or cyanate (OCN⁻). Preferably the anion is halide, mostpreferably, chloride. Most surprisingly, it has been found that the useof ionic liquids containing other anions such as carboxylates, whichhave previously been shown to dissolve cellulose, does not lead to thesatisfactory hydrolysis of cellulose.

Preferred cations which may be present in ionic liquids for use in themethod of the present invention are disclosed in U.S. Pat. No.6,284,599. The cations of the ionic liquid are preferably cyclic,preferably containing an optionally substituted cation selected frompyridinium, pyridazinium, pyrimidinium, pyrazinium, imidazolium,pyrazolium, oxazolium, triazolium, thiazolium, piperidinium,pyrrolidinium, quinolinium and isoquinolinium, and preferably correspondin structure to a formula selected from the group consisting of:

wherein R¹ and R² are independently a C₁-C₆ alkyl group or a C₁-C₆alkoxyalkyl group, and R³, R⁴, R⁵, R⁶, R⁷, R⁸ and R⁹ (R³-R⁹), whenpresent, are independently selected from a hydrido, a C₁-C₆ alkyl, aC₁-C₆ alkoxyalkyl group or a C₁-C₆ alkoxy group. More preferably, bothR¹ and R² groups are C₁-C₄ alkyl, with one preferably being methyl, andR³ -R⁹, when present, are preferably hydrido. Exemplary C₁-C₆ alkylgroups include methyl, ethyl, propyl, iso-propyl, butyl, sec-butyl,iso-butyl, pentyl, iso-pentyl, hexyl,2-ethylbutyl, 2-methylpentyl andthe like. Corresponding C₁-C₆ alkoxy groups contain the above C₁-C₆alkyl group bonded to an oxygen atom that is also bonded to the cationring. An alkoxyalkyl group contains an ether group bonded to an alkylgroup, and here contains a total of up to six carbon atoms. It is to benoted that there are two isomeric 1,2,3-triazoles. It is preferred thatall R groups not required for cation formation be hydrido.

The phrase “when present” is used herein in regard to substituent Rgroups because not all cations have all of the numbered groups. All ofthe contemplated cations contain at least four R groups, although R²need not be present in all cations.

A cation that contains a single five-membered ring that is free offusion to other ring structures is more preferred, for example, animidazolium cation of Formula A is particularly preferred, wherein R¹,R², and R³-R⁵, are as defined before; preferably the anion of the ionicliquid is one of those given above, especially a halogen orpseudohalogen.

A 1,3-di-(C₁-C₆ alkyl or C₁-C₆ alkoxyalkyl)-substituted-imidazolium ionis a more particularly preferred cation; i.e., an imidazolium cationwherein R³-R⁵ of Formula A are each hydrido, and R¹ and R² areindependently each a C₁-C₆ -alkyl group or a C₁-C₆ alkoxyalkyl group.More preferably still one of the 1,3-di-C₁-C₆ alkyl groups R¹ or R² ismethyl.

A 1-(C₁-C₆ -alkyl)-3-(methyl)-imidazolium [C_(n)-mim, where n=1-6]cation is most preferred. A most preferred cation is illustrated by acompound that corresponds in structure to Formula B, below, whereinR³-R⁵ of Formula A are each hydrido and R¹ is a C₁-C₆ alkyl group or aC₁-C₆ alkoxyalkyl group.

Also preferred are pyridinium cations analogous to the imidazoliniumcations discussed above, for example 1-C₁₋₆alkylpyridinium cations.Thus, especially preferred cations are a 1-methyl-3-C₁₋₆alkylimidazoliumor a 1-C₁₋₆alkylpyridiniium cation. Preferably a C₁₋₆ alkyl group is aC₁₋₄ alkyl group, for example a methyl or ethyl group.

Typically, cellulose is admixed with the ionic liquid in an amount of atleast 5% by weight, preferably in an amount of 5 to about 35% weight,for example 5 to 25% percent by weight, especially 10 to about 25%percent by weight.

Stoichiometrically, the hydrolysis reaction requires the presence of onemole equivalent of water for each monomer unit in the cellulose.Cellulose itself contains a certain amount of water, the exact amountdepending upon the source and the physical form of the cellulose;usually, prepared cellulose contains at least 10-15% by weight of water.Further water is added to the reaction mixture if aqueous acid is used.However, excessively high amounts of water in the reaction mixture mayresult in either reduced solubility of the cellulose in the ionicliquid, and/or reduced conversion of cellulose to water-solublehydrolysis products. Preferably the total water content of the reactionsystem is such that the weight ratio of water to cellulose is from 1:1to 1:20, preferably from 1:5 to 1:15, especially about 1:10.

If desired, an additional co-solvent which is compatible with the ionicliquid may be present in the reaction mixture along with the celluloseand the ionic liquid, for example to modify the viscosity of thereaction mixture. Suitable solvents include non-basic polar solvents,for example dimethylsulfoxide, dimethylformamide and sulfolane.

As stated above, the cellulose may be either refined or derived directlyfrom cellulosic biomass, municipal waste or other sources. Thewater-soluble products of the hydrolysis of cellulose include (a) watersoluble oligosaccharides having 3 to 10 D-glucose units; (b) cellobiose;(c) monosaccharides such as glucose and fructose; and (d) glucosederivatives such as levoglucosan, levoglucosenone, levulinic acid,formic acid, 2-furfural, 5-hydroxymethyl-2-furfural,5-methyl-2-furfural, 2,3-butanedione, glycolaldehyde, glyoxal,2-furyl-hydroxymethylketone and pyruval. In general, the most desiredproducts obtainable using the process of the invention are glucoseand/or its water soluble oligomers.

When the conversion of cellulose to products has proceeded to therequired extent, the reaction mixture may be worked up by any suitablemethod. For example, water or another solvent, for example an alcohol,e.g. ethanol, may be added to the reaction mixture in order toprecipitate any residual cellulose or any insoluble hydrolysis products.Where the ionic liquid is hydrophilic and water is added, an aqueoussolution of the ionic liquid and the water-soluble hydrolysis productsmay be produced. Preferably, the ionic liquid used in the process of theinvention is at least partially recovered and reused in the process ofthe invention. If necessary, any solid material, for example comprisingundissolved or unconverted cellulose and/or water insoluble cellulosehydrolysis products, may be separated by any suitable method, and ifdesired, recycled back to the start of the process.

Alternatively, the reaction mixture or any fraction thereof may be useddirectly in any subsequent step required to process the products of thereaction.

In a preferred embodiment of the process of the invention, subsequentprocessing of the products formed is carried out to produce loweralcohols, particularly ethanol, suitable for use as a biofuel. Thus, ina further embodiment, the invention provides a process for thepreparation of one or more alcohols, which comprises admixing cellulosewith an ionic liquid capable of solvating or dissolving at least some ofthe cellulose, said ionic liquid being a compound comprised solely ofcations and anions and which exists in a liquid state at a temperatureat or below 150° C., and in which the anions are selected from halideand cyanate; and treating the resulting solvate or solution with an acidin the presence of water, said acid having a pKa in water of less than 2at 25° C., and converting at least part of the resulting product intoone or more alcohols. The water-soluble cellulose hydrolysis productsmay for example be converted into alcohols by fermentation. Thefollowing Examples illustrate the invention.

EXAMPLE 1

Fibrous cellulose (2.5 g, Aldrich) was added to molten1-ethyl-3-methylimidazolium chloride ([C₂-mim]Cl) (25 g, Fluka) at 90°C. in a 250 mL round-bottomed flask equipped with an over-head stirrerand half-moon paddle and stirred at 100 rpm for approximately two hoursto give a viscous, homogeneous solution of dissolved cellulose. 1 mLconc. HCl was added to the stirred cellulose solution, which was thenstirred at 400 rpm for 30 minutes. Over this period, the break-down ofthe long-chain cellulose polymer could be observed directly by thereduction in viscosity of the reaction mixture. A sample was taken fromthe reaction mixture, quenched with distilled water, and analysed. 45%of the cellulose had been converted into water-soluble products havingglucose end-groups.

EXAMPLE 2

Using the same method as Example 1, 2.5 g of fibrous cellulose wasdissolved in 25 g of [C₂-mim]Cl (Fluka, technical grade), thetemperature was then reduced to 72° C., and 1 mL concentrated HCl wasadded. After 80 minutes, a sample of the reaction mixture was quenchedinto water and analysed. 37% of the cellulose had been converted intowater-soluble products having glucose end-groups.

EXAMPLE 3

25 g of [C₂-mim]Cl (Fluka, technical grade) was heated at 90° C. untilmolten. 2.5 g of fibrous cellulose (Aldrich) was added slowly to thehot, stirred ionic liquid and agitated until at least the majority ofthe cellulose power had dissolved. 1 mL of 50 wt % aq. H₂SO₄ was addedslowly to the reaction mixture, ensuring complete dispersion to minimisecellulose reprecipitating on contact with localised aqueous regions inan inhomogeneous mixture. After 30 minutes, a sample of the reactionmixture was quenched into water and analysed. 58% of the cellulose hadbeen converted into water-soluble products having glucose end-groups.

EXAMPLE 4

Using the same method as in Example 3, 2.5 g of fibrous cellulose wasdissolved in 25 g of [C₂-mim]Cl (Fluka, technical grade) and thenreacted with 1 mL conc. HNO3. After 30 minutes, a sample of the reactionmixture was quenched into water and analysed. 41% of the cellulose hadbeen converted into water-soluble products having glucose end-groups.

EXAMPLE 5

Using the same method as in Example 3, 2.5 g of fibrous cellulose wasdissolved in 25 g of [C₂-mim]Cl (Fluka, technical grade) and thenreacted with 1 mL 75 wt % aq H₂SO₄. After 30 minutes, a sample of thereaction mixture was quenched into water and analysed. 45% of thecellulose had been converted into water-soluble products having glucoseend-groups.

EXAMPLE 6

Using the same method as in Example 3, 1.25 g of fibrous cellulose wasdissolved in 25 g of [C₂-mim]Cl (Fluka, technical grade) and thenreacted with 3 mL 50 wt % aq H₂SO₄. After 30 minutes, a sample of thereaction mixture was quenched into water and analysed. All of thecellulose had been converted into water-soluble products having glucoseend-groups.

EXAMPLE 7

Using the same method as in Example 3, 2.2 g of fibrous cellulose wasdissolved in 25 g of [C₂-mim]Cl (Fluka, technical grade) and thenreacted with 1 mL aq trifluoromethane sulfonic acid (1.95 mL dissolvedin 10 mL water). After 30 minutes, a sample of the reaction mixture wasquenched into water and analysed. 39% of the cellulose had beenconverted into water-soluble products having glucose end-groups.

EXAMPLE 8

Raw biomass from Cortaderia was milled and sieved through a 0.5 mm mesh.0.5 g was then added to a mixture of 10 g of 1-ethyl-3-methylimidazoliumchloride and 0.5 mL concentrated sulfuric acid at 100° C. After stirringat this temperature for 5 minutes, a sample was taken, and analysed viareaction with 3,5-dinitrosalicylic acid. This indicated a yield of 11%of water soluble products having glucose end groups.

EXAMPLE 9

2.23 g of fibrous cellulose were added to 25 g of 1-ethylpyridiniumchloride at 90° C., and stirred until dissolution was observed. 1 mL of50 wt % aqueous sulfuric acid was added. After 12 minutes, analysisindicated a conversion of 10% to the desired products.

EXAMPLE 10 Comparative

0.2 g of fibrous cellulose were dissolved in 2 g of1-ethyl-3-methylimidazolium acetate at 90° C. 0.1 mL of conc. HCl wereadded to this and the reaction mixture was sampled after 5, 15, 30, 60and 90 minutes. No glucose products were detected by refractive indexHPLC, or by DNS analysis.

EXAMPLE 11 Comparative

0.25 g of fibrous cellulose were dissolved in 2 g of1-ethyl-3-methylimidazolium acetate at 110° C. 0.3 mL of conc. HCl wereadded to this and the reaction mixture was sampled after 5, 15, 30, 90and 180 minutes. No glucose products were detected by refractive indexHPLC, or by DNS analysis.

The invention claimed is:
 1. A process for the preparation of one ormore alcohols, the process comprising admixing cellulose with an ionicliquid capable of solvating or dissolving at least some of thecellulose, wherein the ionic liquid is a compound comprised solely ofcations and anions, and is a liquid at a temperature of 150° C. orbelow, wherein the anions are selected from the group consisting ofhalide and cyanate; reacting the resulting solvate or solution with anacid in the presence of water, said acid having a pKa in water of lessthan 2 at 25° C.; and converting at least part of the resultinghydrolysis products into one or more alcohols.
 2. The process of claim1, wherein said ionic liquid is capable of dissolving at least some ofthe cellulose.
 3. The process of claim 1, wherein the acid has a pKa inwater of 0 or less at 25° C.
 4. The process of claim 1, wherein the acidis selected from the group consisting of hydrogen halides, sulfuricacid, nitric acid, halosulfonic acids, tetrafluoroboric acid,heteropolyacids, aryl- and alkyl-sulfonic acids, and halogenated alkyl-and arylsulfonic acids.
 5. The process of claim 4, wherein the acid isp-toluenesulfonic acid, trifluoromethanesulfonic acid,trichloromethanesulfonic acid, hydrochloric acid, hydrobromic acid,hydriodic acid, tetrafluoroboric acid, or sulfuric acid.
 6. The processof claim 5, wherein the acid is sulfuric acid or hydrochloric acid. 7.The process of claim 1 , wherein the reaction with the acid is carriedout at a temperature in the range of from 50 to 200° C.
 8. The processof claim 1, wherein the cation of the ionic liquid is selected from thegroup consisting of pyridinium, pyridazinium, pyrimidinium, pyrazinium,imidazoliuni, pyrazolium, oxazolium, triazolium, thiazolium,piperidinium, pyrrolidinium, quinolinium and isoquinolinium.
 9. Theprocess of claim 8, wherein the cation of the ionic liquid has astructure selected from the group consisting of:

wherein R¹ and R² are independently a C₁-C₆ alkyl group or a C₁-C₆alkoxyalkyl group, and R³, R⁴, R⁵, R⁶, R⁷, R⁸ and R⁹ (R³-R⁹), whenpresent, are independently selected from the group consisting ofhydrido, C₁-C₆ alkyl, C₁-C₆ alkoxyalkyl and C₁-C₆ alkoxy.
 10. Theprocess of claim 9, wherein both R¹ and R² are C₁-C₆ alkyl, and R³-R⁹,when present, are hydrido.
 11. The process of claim 9, wherein thecation of the ionic liquid is an imidazolium cation or a pyridiniumcation.
 12. The process of claim 11, wherein R³-R⁷ are each hydrido. 13.The process of claim 12, wherein the cation is a 1-methyl-3-C₁₋₆alkylimidazolium or a 1-C₁₋₆alkylpyridiniium cation.
 14. The processof claim 1, wherein the anion is a halide.
 15. The process of claim 14,wherein the anion is chloride.
 16. The process of claim 1, wherein thecellulose is admixed with the ionic liquid in an amount of from 5 to 35%weight.
 17. The process of claim 1, wherein water is present in thereaction system in a weight ratio of water to cellulose from 1:1 to1:20.
 18. The process of claim 17, wherein the weight ratio of water tocellulose is from 1:5 to 1:15.